1. Field of the Invention
The present disclosure relates to a flat panel display, and more particularly, to a flat panel display having a low reflective black matrix and a method for manufacturing the same.
2. Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP) and the electroluminescence device (or EL). Especially, a high quality flat panel display adopting the low temperature poly silicon (or, LTPS) as the channel device is increasingly required.
The electroluminescence display device is categorized in the inorganic light emitting diode display device and the organic light emitting diode display device according to the luminescence material. As a self-emitting display device, the electroluminescence display device has the merits those the response speed is very fast, the brightness is very high and the view angle is large. Especially, because of high energy efficiency, low current leakage, and easiness of the color reproducing using the current control, the organic light emitting diode display is increasingly required.
FIG. 1 is a diagram illustrating the structure of the organic light emitting diode. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole from the anode and the electron from the cathode are recombined at the emission layer EML. The organic light emitting diode display can represent the video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer ELM of the organic light emitting diode as shown in FIG. 1.
The organic light emitting diode display (or OLED) using the organic light emitting diode can be categorized in the passive matrix type organic light emitting diode display (or PMOLED) and the active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows the video data by controlling the current applying to the organic light emitting diode using the thin film transistor (or TFT).
FIG. 2 is the exemplary circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED). FIG. 3 is the plane view illustrating the structure of AMOLED using the thin film transistor according to the related art. FIG. 4 is the cross sectional view along the cutting line I-I′ in FIG. 3, for illustrating the structure of the bottom emission type AMOLED according to the related art. FIG. 5 is the cross sectional view along the cutting line I-I′ in FIG. 3, for illustrating the structure of the top emission type AMOLED according to the related art. As the differences between the bottom emission type and the top emission type are not shown in the plane views, the FIG. 3 is used commonly.
Referring to FIGS. 2 and 3, the active matrix organic light emitting diode display comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLED connected to the driving thin film transistor DT. The scan line SL, the data line DL and the driving current line VDD are formed on the substrate SUB to define the pixel area. The organic light emitting diode OLED is formed within the pixel area to define the light emitting area.
The switching thin film transistor ST is formed where the scan line SL and the data line DL is crossing. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OD disposed at the pixel selected by the switching thin film transistor ST.
The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLED. The organic light emitting layer OLE is inserted between the anode electrode ANO and the cathode electrode CAT. The cathode electrode CAT is connected to the base voltage (or, ground voltage) VSS. There is a storage capacitance Cst is disposed between the gate electrode DG of the driving thin film transistor DT and the driving current line VDD or between the gate electrode DG of the driving thin film transistor DT and the drain electrode DD of the driving thin film transistor DT.
Referring to FIG. 4, we will explain about the bottom emission type organic light emitting diode display in detail. On the substrate SUB of the active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the drain contact hole DH penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
As mentioned above, the substrate SUB having the thin film transistors ST and DT has uneven surface and level differences because there are many elements. It is preferable for the organic light emitting layer OL to be formed on even surface to ensure the uniformly light emitting distribution over all area of the organic light emitting layer OL. Therefore, in order to make the surface of the substrate SUB smooth, the over coat layer OC (or, the planar layer) is deposited over the substrate SUB.
On the over coat layer OC, an anode electrode ANO of the organic light emitting diode OLE is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT via the pixel contact hole PH formed at the over coat layer OC and the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank (or ‘bank pattern’) BN is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL and VDD, for defining the light emitting area. The exposed portion of the anode electrode ANO by the bank BN would be the light emitting area. On the anode electrode ANO exposed from the bank BN, the organic light emitting layer OL is formed. On the organic light emitting layer OL, the cathode electrode CAT is formed.
On the substrate SUB having the cathode electrode CAT, a spacer SP may be disposed. It is preferable that the spacer SP is disposed on the bank BM included in the non-light emitting area. The encap substrate ENC covers and is laminated on the upper side of the lower substrate SUB with the spacer SP between them. To join the encap substrate ENC and the lower substrate SUB, an adhesive layer or an adhesive material (not shown) may be inserted between them.
In the case of the bottom emission type and full-color AMOLED, the light emitted from the organic light emitting layer OL is radiated toward the lower substrate SUB. Therefore, it is preferable that the color filter CF is disposed between the overcoat layer OC and the passivation layer PAS, and the anode electrode ANO is made of a transparent conductive material. Further, it is preferable that the cathode electrode CAT includes a metal material having good reflective property for reflecting the light from the organic light emitting layer OL toward the lower substrate SUB. In addition, the organic light emitting layer OL may include an organic material generating the white light. The organic light emitting layer OL and the cathode electrode CAT may be deposited as covering the whole surface of the lower substrate SUB.
Hereinafter, referring to FIG. 5, we will explain about the top emission type full color organic light emitting diode display. The basic structure is very similar with the bottom emission type. Therefore, the explanation about the same structure may not be mentioned. For the case of the top emission type, the light from the organic light emitting layer OL is radiated toward the encap substrate ENC disposed over the lower substrate SUB. Therefore, it is preferable that the anode electrode ANO has a good reflective property and the cathode electrode CAT is made of a transparent conductive material.
In order to reproduce/represent the full color, the organic light emitting layer OL in each pixel may include any one color among red, green and blue which is disposed in each pixel. The cathode electrode CAT may be deposited as covering the whole surface of the lower substrate SUB. Otherwise, the organic light emitting layer OL may include an organic material generating the white light, and a color filter CF may be disposed on the organic light emitting layer OL or on the cathode electrode CAT. Here, in convenience, the figure shows that the color filter CF is disposed on the cathode electrode CAT. The color filter CF may be arrayed in the order of red R, green G and blue B.
For the bottom emission type, the user may see the video information at the lower substrate SUB side. On the contrary, for the top emission type, the user may see the video information at the encap substrate ENC side. Therefore, lights incident from the outside may be reflected by outside surface of the lower substrate SUB or the encap substrate ENC so that the reflective light may hinder the observation feeling and quality of the user. Especially, in the case that black matrix are disposed between each pixel, the light may be further reflected by the surface of the black matrix so that the video quality may be worse.
To prevent these drawbacks, in one method, a λ/4 polarization sheet may be attached on the display panel at the observation side. For example, in the bottom emission type, the polarization plate may be attached on the outer surface of the lower substrate SUB. In the top emission type, the polarization plate or sheet may be attached on the outer surface of the encap substrate ENC. However, with the polarization sheet or plate, the overall light transmissivity may be degraded. As the whole brightness of the display may be lowered, higher power consumption may be required for ensuring enough brightness.